Portable Firefox 3.5 (hangs 2-3 seconds every 2-3 mins)

As i mentioned in the subject the portable firefox 3.5 keep hanging 2-3 seconds every 2-3 mins. I am running it on my sandisk pendrive. Before i make a switch to firefox 3.5, i was using firefox 3.0.x and its works fine for me. i disabled the session restore through the about:config but somehow the "undo closed tab" feature was not disabled. I believe that is component which writes to the disk every 1-2 mins about the current sessions. Anyone have any idea on how could i disable this feature? or please share with me if u have other workaround to fix the problem.

ps: "undo closed tab" feature will be disabled when session restore is set to false on firefox 3.0.x but not with firefox 3.5